1. Field of the Invention
The present invention relates to a switching element and a switching stage, each of which has two signal inputs and two signal outputs, and further relates to a switching system having a number of switching elements arranged in the form of a matrix.
2. Description of the Prior Art
In the past, various solutions for a switching network have been proposed. Such switching networks have a multiplicity of inputs and outputs, in which arrangement the data streams or message streams are switched through from one of the inputs to at least one fixed output. A message stream directed to one of the inputs of the switching network is, thus, forwarded in such a manner that it leaves it again at one of the outputs. In such known switching network arrangements, the switching network units themselves are constructed of a multiplicity of discrete components; for example, transistors and resistors.
Such switching networks are implemented as switching network matrices; i.e., the switching points implemented by the switching network can be described by the elements of a matrix and, thus, have a matrix-like structure. In the implementation of switching network matrices for high-bit-rate data signals, the data signals to be switched are loaded by the very high line capacities caused by data signal lines or switching elements within the switching network arrangement. As such, the degradation of the data signals increases considerably at high data transmission rates which leads to a limitation of the size of the switching network matrix which can be implemented.
To implement larger switching network matrices, switching arrangements are known in which an nxc3x97m switching network matrix is formed by cascading mn: 1 multiplexer units. Dividing the complex switching matrix to be implemented into a number of smaller sub-matrices in this manner makes it possible to implement larger switching network matrices. However, the use of a number of weaker line drivers for the multiplicity of small sub-matrices considerably increases the degradation of the data signals, i.e. the power loss, and the complexity of the drive to the switching arrangement.
An object of the present invention, therefore, is to improve the circuit implementation of switching elements or switching network systems.
The switching element of the present invention thus includes first and second transistors, the base terminals of which are connected to one another and are connected to a drive unit. Furthermore, the emitter terminals of the first and second transistor form the two signal inputs to which a current source can be connected in each case. The collector terminals of the first and second transistor form the two signal outputs. According to the teachings of the present invention, the switching element includes a differential common-base transistor pair which makes possible to provide a high-impedance drive to the base of the first and second transistor. Due to the connection between the two base terminals, the base charge necessary for high-frequency operation can flow between these two terminals; i.e., the high-frequency base currents, which are comparatively high with respect to the direct-current case, do not need to be provided by the drive unit so that the power loss of the drive unit can be kept low. The switching function of the switching element is implemented by the drive unit, in which the application of a constant base potential to the base terminals of both the first and second transistors turns on one of the two transistors which, in turn, passes through the entire data signal stream as well as the auxiliary currents generated by the current sources. By providing current sources, especially constant-current sources, connected to the emitter terminals in accordance with the present invention, the transistor not turned on by the drive unit additionally becomes conductive and, thus, conducts the auxiliary current generated by the respective constant-current source. As a result, the resultant signal excursion on the input signal lines is reduced and the influence of capacitive loads at the input signal lines is distinctly reduced.
According to a further embodiment of the present invention, an output stage, or switching stage, having two signal inputs and two signal outputs is provided which has a first and second transistor, the base terminals of which are connected to one another and are connected to a drive unit or a reference potential. In this arrangement, the emitter terminals of the first and second transistor form the two signal inputs to which, in each case, one current source can be connected. The collector terminals of the first and second transistor form the two signal outputs to which a reference potential is, in each case, connected via a resistor. The output stage is implemented by a transistor pair operated in differential common base, to the collector terminals of which a reference potential is connected via a resistor. The output stage thus operates as a current/voltage converter. In this arrangement, the reference potential required for turning on can be provided at low power, due to the coupling of the base terminals of the first and second transistor according to the present invention, since the required base charge can flow between the two base terminals of the transistor pair.
Also, according to the present invention, the two signal inputs of the output stage are additionally connected to two outputs of an input circuit constructed as a differential amplifier, as a result of which a point-to-point transmission link can be implemented with low power loss. The data transmitted in the form of voltage levels is converted by the input circuit constructed as a differential amplifier into signal streams which are alternately present at the two signal inputs. The data is thus transmitted from the input unit to the output unit in the form of signal streams, in which case the output unit can be easily arranged remotely from the input unit.
Furthermore, the advantage of the switching system of the present invention, having a number of switching elements arranged in the form of a matrix, can be seen in the fact that in each row of the matrix. the two signal inputs of one of the switching elements arranged in a row, are connected to a first and second input signal line to which a current source is connected in each case. In each column of the matrix, the two signal outputs of one of the switching elements arranged in a column are additionally connected to a first and second output signal line. Moreover, a further current source is connected in each case to the first and second output signal line, and the first and second output signal line are connected to the two signal inputs of one output stage. Such a switching system or switching network matrix can be implemented with extremely low power loss since, with the aid of the current sources connected to the first and second input signal lines of a row of the switching matrix, the first and second transistor of the switching element arranged in the respective row of the matrix, at which a switching potential is present with the aid of the drive unit, in each case turns on. The differential current generated by the input unit and representing the data is then alternately forwarded to the output stage via the first or second transistor arranged in the switched switching element, additionally to the currents generated by the current sources. The excursion on the first and second input signal line is adjusted by the presence of a constant current, generated by the current sources, at the first and second input signal line in accordance with the teachings of the present invention. This considerably reduces the influence of the capacitive loads. This also makes it possible to implement larger switching networks without increasing the complexity of the circuit implementation or, respectively, of the drive circuits.
According to a further embodiment of the switching system of the present invention, a further current source is, in each case, connected to the first and second output signal line. This makes it possible in a particularly advantageous manner to design the transmission of the signal currents via the first and second output signal lines, in combination with an output stage, to have a low power loss.
Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and the Drawings.